The present invention relates to load indicating members and load indicating fasteners and further relates to methods and apparatuses for making and using load indicating members and load indicating fastening members. More particularly, the present invention relates to a method of measuring a load in the load indicating member, a load measuring device using the method of measuring the load, a fastener tightening tool for tightening a load indicating fastener member using the method of measuring, a load indicating member and a load indicating fastener for use in conjunction with the method of measuring, a method of making the load indicating fastener, and a method of tightening the load indicating fastener.
In many operations, it is desirable to determine the amount of load experienced by a longitudinally stressed member. This information is particularly useful when the longitudinally stressed member is a fastener since the measurement of the longitudinal stress provides a verification of the existence of a proper joint.
A prior technique which has been used to indicate the amount of longitudinal stress experienced by a fastener is to provide a load indicating feature to the fastener itself. This may be done by interconnecting one end of an elongated member to a portion of the fastener prior to the installation of the fastener. The elongated member extends parallel to the fastener and is unaffected by the elastic deformation of the fastener in response to longitudinal stress. The free end of the elongated member therefore provides a reference for indicating the elongation of the fastener in response to the longitudinal stress.
Typically, the elongated member is a pin received within an elongated bore disposed longitudinally of the fastener and extended from the head of the fastener and partially through the shank of the fastener. One end of the pin is interconnected with the shank of the fastener at the base of the bore by means, for example, of adhesives, threads, or an interference fit.
The various prior load indicating members and load indicating fasteners differed greatly in structure as well as in the methods and apparatuses with which they were used to provide an indication of the elongation of the load indicating member or fastener.
For example, in the U.S. Pat. No. 3,602,186 issued Aug. 31, 1971 to Charles H. Popenoe, a fastener of this type is taught wherein the free end of the pin is disposed within a recess formed in the head of the fastener and is submerged in a dark fluid filling the recess. A transparent or translucent window is set in the recess to seal against leakage of the fluid and to provide a view of the free end of the pin. When the fastener is in an unstressed condition, the free end of the pin rests flush against the window. As the fastener is stressed longitudinally, the pin moves away from the window, changing the color which is seen through the window and, thereby, providing a visual indication of the elongation of the shank of the fastener. If the fastener later becomes loosened, the indicator window reverts to its original color, providing a visual warning of the loosened condition. A variation of this load indicating fastener is taught by U.S. Pat. No. 3,799,108 issued Mar. 26, 1974 to Jack E. Mosow.
In U.S. Pat. No. 2,413,797 issued Jan. 7, 1947 to Albert R. Stone, a fastener of this type is taught wherein the free end of the pin is provided with a head spaced a predetermined distance away from a preselected surface formed on the fastener. When the fastener is tightened to a preselected longitudinal stress, such as to result in a predetermined amount of elongation, the head of the pin engages the preselected surface of the fastener to provide an indication that the desired elongation has been achieved.
In the similar fastener taught by U.S. Pat. No. 3,248,923 issued May 3, 1966 to Richard H. Blakeley, the free end of the pin is provided with a first predetermined surface which becomes coplanar with a second predetermined surface of the fastener when a preselected stress level is reached. The fastener is calibrated by first being tensioned longitudinally to the desired stress level. Then, either the free end surface of the pin and the adjacent end surface of the head of the fastener are ground simultaneously to become coplanar or, alternatively, the pin is pressed into a reduced diameter bore in the base of the elongated bore until the free end surface of the pin is flush with the adjacent end surface of the head of the fastener. In the unstressed condition of the fastener, the free end of the pin protrudes above the end surface of the head of the fastener. A similar fastener is taught by German Patent-schrift No. 745,925 published Dec. 23, 1943.
In U.S. Pat. No. 3,812,758 issued May 28, 1974 to Robert B. Bessler, Jr. and U.S. Pat. No. 3,823,639 issued July 16, 1974 to Theodore Liber, a cross bar is formed at the free end of the pin which indicates the elongation of the shank of the fastener relative to the elongated pin by torsional deflection.
While the above-referenced seven patents each teach an acceptable structure for providing a visible indication that a preselected amount of elongation has occurred, none of these patents provides for a numerical measurement of the exact amount of elongation which has occurred and none provides for the achievement of different amounts of preselected elongation for alternative installations of the same fastener. Furthermore, none of these patents provides for remote monitoring of the fasteners either during or after installation.
Still another load indicating member and load indicating fastener is taught by U.S. Pat. No. 2,503,141 issued Apr. 4, 1950 to Albert R. Stone. The fastener is modified by providing an extra threaded boss extending from the head of the fastener in the opposite direction than the shank. The free end of the pin is substantially flush with the end of the threaded boss in the unstressed condition. A load measuring device is threaded onto the threaded boss. A plunger reciprocates relative to the load measuring device and engages the free end of the pin. The load measuring device detects when the plunger has moved a predetermined amount relative to the end of the boss such as to indicate that a preselected stress level has been reached. A similar fastener and a similar load measuring device are taught by U.S. Pat. No. 3,943,819 issued Mar. 16, 1976 to Charles S. Charron.
Both Stone '141 and Charron require a substantial modification to the head of the fastener which results in the load indicating fastener protruding further than an unmodified fastener. It is therefore necessary to manufacture the load indicating fasteners independently since it is not practical to modify existing bolts to the configurations taught by either of these patents. Furthermore, neither Stone nor Charron provide inexpensive, quick, and accurate means for aligning the load measuring device with the load indicating member when a measurement is being taken.
Yet other load measuring devices, load indicating members, and load indicating fasteners using pins or similar elongated members to indicate the elongation of the load indicating member or fastener are disclosed in U.S. Pat. No. 2,600,029 issued June 10, 1952 to Albert R. Stone, U.S. Pat. No. 3,908,508 issued Sept. 30, 1975 to William J. Payne; U.S. Pat. No. 3,987,668 issued Oct. 26, 1976 to Charles H. Popenoe; and U.S. Pat. No. 4,114,428 issued Sept. 19, 1978 to Charles H. Popenoe.
While each of the various prior load indicating members, and load measuring devices cited above provides its own load indicating fasteners, advantages in terms of accuracy, ease of manufacture, or ease of reading, none combines all of these advantages in a single structure. For example, none provides a compact load indicating member which may be rapidly and reliably interconnected and accurately aligned with a load measuring device. None provides an accurate load indicating fastener which may be rapidly and inexpensively manufactured from existing fasteners. None provides for the protection of an accurate load indicating member from contamination after installation and between readings. None provides a load measuring device providing an accurate continuous measurement of the elongation of a fastener during installation. Finally, none provides a fastener tightening tool responsive to a load indicating member to provide a precise automatic tightening operation for a fastener.